Right, if I want to rely on Finagle's RPC capabilities, e.g. load balancing, routing, tracing, etc, then my microservice must be JVM based and use the Finagle library. This is a standalone proxy deployed as a 'sidecar' app alongside my service. This allows me to implement the microservice in any language I want and get some of the features listed previously.

Of course you now have another moving part in your system :)

That's basically right. linkerd is a proxy that deals with a bunch of operational things like service discovery, metrics/observability, tracing, load balancing, etc. It's a proxy so that you can deploy one solution across all languages/frameworks without having to modify your code to deal with these things.